Mitochondria are the main energy source in cells of higher organisms, and these organelles provide direct and indirect biochemical regulation of a wide array of cellular respiratory, oxidative and metabolic processes. These include electron transport chain (ETC) activity, which drives oxidative phosphorylation to produce metabolic energy in the form of adenosine triphosphate (ATP), and which also underlies a central mitochondrial role in intracellular calcium homeostasis.
Mitochondrial ultrastructural characterization reveals the presence of an outer mitochondrial membrane that serves as an interface between the organelle and the cytosol, a highly folded inner mitochondrial membrane that appears to form attachments to the outer membrane at multiple sites, and an intermembrane space between the two mitochondrial membranes. The subcompartment within the inner mitochondrial membrane is commonly referred to as the mitochondrial matrix. (For a review, see, e.g., Ernster et al., 1981 J. Cell Biol. 91:227s.) The cristae, originally postulated to occur as infoldings of the inner mitochondrial membrane, have recently been characterized using three-dimensional electron tomography as also including tube-like conduits that may form networks, and that can be connected to the inner membrane by open, circular (30 nm diameter) junctions (Perkins et al., 1997, Journal of Structural Biology 119:260). While the outer membrane is freely permeable to ionic and non-ionic solutes having molecular weights less than about ten kilodaltons, the inner mitochondrial membrane exhibits selective and regulated permeability for many small molecules, including certain cations, and is impermeable to large (>˜10 kDa) molecules.
Altered or defective mitochondrial activity, including but not limited to failure at any step of the ETC, may result in catastrophic mitochondrial collapse that has been termed “permeability transition” (PT) or “mitochondrial permeability transition” (MPT). According to generally accepted theories of mitochondrial function, proper ETC respiratory activity requires maintenance of an electrochemical potential (ΔΨm) in the inner mitochondrial membrane by a coupled chemiosmotic mechanism. Altered or defective mitochondrial activity may dissipate this membrane potential, thereby preventing ATP biosynthesis and halting the production of a vital biochemical energy source. In addition, mitochondrial proteins such as cytochrome c may leak out of the mitochondria after permeability transition and may induce the genetically programmed cell suicide sequence known as apoptosis or programmed cell death (PCD).
MPT may result from direct or indirect effects of mitochondrial genes, gene products or related downstream mediator molecules and/or extramitochondrial genes, gene products or related downstream mediators, or from other known or unknown causes. Loss of mitochondrial potential therefore may be a critical event in the progression of diseases associated with altered mitochondrial function, including degenerative diseases.
Mitochondrial defects, which may include defects related to the discrete mitochondrial genome that resides in mitochondrial DNA and/or to the extramitochondrial genome, which includes nuclear chromosomal DNA and other extramitochondrial DNA, may contribute significantly to the pathogenesis of diseases associated with altered mitochondrial function. For example, alterations in the structural and/or functional properties of mitochondrial components comprised of subunits encoded directly or indirectly by mitochondrial and/or extramitochondrial DNA, including alterations deriving from genetic and/or environmental factors or alterations derived from cellular compensatory mechanisms, may play a role in the pathogenesis of any disease associated with altered mitochondrial function. A number of degenerative diseases are thought to be caused by, or to be associated with, alterations in mitochondrial function. These include Alzheimer's Disease (AD); diabetes mellitus; Parkinson's Disease; Huntington's disease; dystonia; Leber's hereditary optic neuropathy; schizophrenia; mitochondrial encephalopathy, lactic acidosis, and stroke (MELAS); cancer; psoriasis; hyperproliferative disorders; mitochondrial diabetes and deafness (MIDD) and myoclonic epilepsy ragged red fiber syndrome. The extensive list of additional diseases associated with altered mitochondrial function continues to expand as aberrant mitochondrial or mitonuclear activities are implicated in particular disease processes.
A hallmark pathology of AD and potentially other diseases associated with altered mitochondrial function is the death of selected cellular populations in particular affected tissues, which results from apoptosis (also referred to as “programmed cell death” or PCD) according to a growing body of evidence. Mitochondrial dysfunction is thought to be critical in the cascade of events leading to apoptosis in various cell types (Kroemer et al., FASEB J. 9:1277-87, 1995), and may be a cause of apoptotic cell death in neurons of the AD brain. Altered mitochondrial physiology may be among the earliest events in PCD (Zamzami et al., J. Exp. Med. 182:367-77, 1995; Zamzami et al., J. Exp. Med. 181:1661-72, 1995) and elevated reactive oxygen species (ROS) levels that result from such altered mitochondrial function may initiate the apoptotic cascade (Ausserer et al., Mol. Cell. Biol. 14:5032-42, 1994).
Thus, in addition to their role in energy production in growing cells, mitochondria (or, at least, mitochondrial components) participate in apoptosis (Newmeyer et al., 1994, Cell 79:353-364; Liu et al., 1996, Cell 86:147-157). Apoptosis is apparently also required for, inter alia, normal development of the nervous system and proper functioning of the immune system. Moreover, some disease states are thought to be associated with either insufficient (e.g., cancer, autoimmune diseases) or excessive (e.g., stroke damage, AD-associated neurodegeneration) levels of apoptosis. For general reviews of apoptosis, and the role of mitochondria therein, see Green and Reed (1998, Science 281:1309-1312), Green (1998, Cell 94:695-698) and Kromer (1997, Nature Medicine 3:614-620). Hence, agents that effect apoptotic events, including those associated with mitochondrial components, might have a variety of palliative, prophylactic and therapeutic uses.
The adenine nucleotide translocator (ANT), a nuclear encoded polypeptide that is a major component of the inner mitochondrial membrane, is responsible for mediating transport of ADP and ATP across the mitochondrial inner membrane. For example, ANT is believed to mediate stoichiometric ATP/proton exchange across the inner mitochondrial membrane, and ANT inhibitors (such as atractyloside or bongkrekic acid) induce MPT under certain conditions. Three human ANT isoforms have been described that differ in their tissue expression patterns and other mammalian ANT homologues have been described. (See, e.g., Wallace et al., 1998 in Mitochondria & Free Radicals in Neurodegenerative Diseases, Beal, Howell and Bodis-Wollner, Eds., Wiley-Liss, New York, pp. 283-307, and references cited therein.) ANT has also been implicated as an important molecular component of the mitochondrial permeability transition pore, a Ca2+-regulated inner membrane channel that, as described above, plays an important modulating role in apoptotic processes.
As inner mitochondrial membrane proteins are believed to possess multiple hydrophobic membrane spanning domains, ANT polypeptides may exhibit, inter alia, poor intracellular solubility, toxic accumulations and/or the formation of inclusion bodies and other deleterious effects on respiratory homeostasis within a host cell due to ANT biological activity. Consequently, those having ordinary skill in the art have heretofore been unable to produce ANT reliably or in sufficient quantities for a variety of uses, such as those provided herein. Because of the significance of mitochondria to respiratory, metabolic and apoptotic processes, and in view of the prominent role played by ANT in these and other mitochondrial acitivities, there is clearly a need for compositions and methods that permit the production of ANT proteins, including ANT fusion proteins; for novel ANT ligands; for methods to identify and isolate ANT proteins; and for methods of identifying and isolating agents that interact with ANT.
The present invention fulfills these needs and provides other related advantages. These and other aspects of the present invention will become evident upon reference to the following detailed description and attached drawings. In addition, various references are set forth below which describe in more detail certain procedures or compositions (e.g., plasmids, vectors, etc.), and are therefore incorporated by reference in their entireties.